Molecular and cellular basis of pulmonary vascular remodeling in pulmonary hypertension

Bone Morphogenetic Protein 4 Promotes Pulmonary Vascular Remodeling in Hypoxic Pulmonary Hypertension

We show that 1 of the type II bone morphogenetic protein (BMP) receptor ligands, BMP4, is widely expressed in the adult mouse lung and is upregulated in hypoxia-induced pulmonary hypertension (PH). Furthermore, heterozygous null Bmp4 lacZ/+ mice are protected from the development of hypoxia-induced PH, vascular smooth muscle cell proliferation, and vascular remodeling. This is associated with a reduction in hypoxia-induced Smad1/5/8 phosphorylation and Id1 expression in the pulmonary vasculature. In addition, pulmonary microvascular endothelial cells secrete BMP4 in response to hypoxia and promote proliferation and migration of vascular smooth muscle cells in a BMP4-dependent fashion. These findings indicate that BMP4 plays a dominant role in regulating BMP signaling in the hypoxic pulmonary vasculature and suggest that endothelium-derived BMP4 plays a direct, paracrine role in promoting smooth muscle proliferation and remodeling in hypoxic PH.

ClC-3 chloride channel is upregulated by hypertrophy and inflammation in rat and canine pulmonary artery

Cl- channels have been implicated in essential cellular functions including volume regulation, progression of cell cycle, cell proliferation and contraction, but the physiological functions of the ClC-3 channel are controversial. We tested the hypothesis that the ClC-3 gene (ClCn-3) is upregulated in hypertensive pulmonary arteries of monocrotaline-treated rats, and upregulated ClC-3 channel aids viability of pulmonary artery smooth muscle cells (PASMCs). Experimental pulmonary hypertension was induced in rats by a single subcutaneous administration of monocrotaline (60 mg kg(-1)). Injected animals developed characteristic features of pulmonary hypertension including medial hypertrophy of pulmonary arteries and right ventricular hypertrophy. Reverse transcriptase-polymerase chain reaction (RT-PCR), immunohistochemistry and Western immunoblot analysis indicated that histopathological alterations were associated with upregulation of the ClC-3 mRNA and protein expression in both smooth muscle cells of hypertensive pulmonary arteries and in cardiac myocytes. RT-PCR analysis of mRNA, extracted from canine cultured PASMCs, indicated that incubation with the inflammatory mediators endothelin-1 (ET-1), platelet-derived growth factor (PDGF), interleukin-1beta (IL-1beta) and tumor necrosis factor alpha (TNF alpha), but not transforming growth factor beta (TGFbeta), upregulated ClC-3 mRNA. Adenovirus-mediated delivery and overexpression of ClC-3 in canine PASMCs improved cell viability against increasing concentrations of hydrogen peroxide (H2O2, range 50-250 microM). In conclusion, upregulation of ClC-3 in rat hypertensive lung and heart is a novel observation. Our functional data suggest that upregulation of ClC-3 is an adaptive response of inflamed pulmonary artery, which enhances the viability of PASMCs against reactive oxygen species.

Increased susceptibility to pulmonary hypertension in heterozygous BMPR2-mutant mice

BACKGROUND

Bone morphogenetic protein receptor-2 (BMPR2)-heterozygous, mutant (BMPR2(+/-)) mice have a genetic trait similar to that of certain patients with idiopathic pulmonary arterial hypertension (IPAH). To understand the role of BMPR2 in the development of IPAH, we examined the phenotype of BMPR2(+/-) mice and their response to inflammatory stress.

METHODS AND RESULTS

BMPR2(+/-) mice were found to have the same life span, right ventricular systolic pressure (RVSP), and lung histology as those of wild-type mice under unstressed conditions. However, when treated with recombinant adenovirus expressing 5-lipoxygenase (Ad5LO), BMPR2(+/-) mice exhibited significantly higher RVSP than wild-type mice. The increase of RVSP occurred in the first 2 weeks after Ad5LO delivery. Modest but significant muscularization of distal pulmonary arterioles appeared in BMPR2(+/-) mice 4 weeks after Ad5LO treatment. Measurement of urinary metabolites of vasoactive molecules showed that cysteinyl leukotrienes, prostacyclin metabolites, and PGE2 were all increased to a similar degree in both BMPR2(+/-) and wild-type mice during 5LO transgene expression, whereas urinary endothelin-1 remained undetectable. Urinary thromboxane A2 metabolites, in contrast, were significantly higher in BMPR2(+/-) than in wild-type mice and paralleled the increase in RVSP. Platelet activation markers, serotonin, and soluble P-selectin showed a trend toward higher concentrations in BMPR2(+/-) than wild-type mice. Cell culture studies found that BMP treatment reduced interleukin-1beta-stimulated thromboxane A2 production in the pulmonary epithelial cell line A549.

CONCLUSIONS

BMPR2(+/-) mice do not develop pulmonary hypertension spontaneously; however, under inflammatory stress, they are more susceptible to an increase in RVSP, thromboxane A2 production, and vascular remodeling than wild-type mice.

Inhibition of hypoxia-induced proliferation and collagen synthesis by vasonatrin peptide in cultured rat pulmonary artery smooth muscle cells

The aim of the present research is to investigate the effects of vasonatrin peptide (VNP) on hypoxia-induced proliferation and collagen synthesis in pulmonary artery smooth muscle cells (PASMCs). Smooth muscle cells isolated from rat pulmonary artery were cultured and used at passages 3-5. Cell proliferation and collagen synthesis were evaluated by cell counts, [(3)H] thymidine and [(3)H] proline incorporation. The results showed that cells exposed to hypoxia for 24 h exhibited a significant increase in [(3)H] thymidine (93%) and [(3)H] proline (52%) incorporation followed by a significant increase in cell number (47%) at 48 h in comparison with the respective normoxic controls. VNP reduced hypoxia-stimulated increase in cell proliferation in a concentration-dependent manner from 10(-8) to 10(-6) mol/L and attenuated hypoxia-induced collagen synthesis ranging from 10(-6) to 10(-5) mol/L, which is similar to but more potent than both ANP and CNP. The action of VNP on PASMCs was mimicked by 8-bromo-cGMP (10(-4) mol/L, the membrane-permeable cGMP analog), and blocked by HS-142-1 (2 x 10(-5) mol/L), the particulate guanylyl cyclase-coupled natriuretic peptide receptor antagonist, or KT-5823 (10(-6) mol/L), the cGMP-dependent protein kinase (PKG) inhibitor. The results suggest that VNP inhibits hypoxia-stimulated proliferation and collagen synthesis in cultured rat PASMCs via particulate guanylyl cyclase-coupled receptors through cGMP/PKG dependent mechanisms.

Hypoxia, leukocytes, and the pulmonary circulation

Data are rapidly accumulating in support of the idea that circulating monocytes and/or mononuclear fibrocytes are recruited to the pulmonary circulation of chronically hypoxic animals and that these cells play an important role in the pulmonary hypertensive process. Hypoxic induction of monocyte chemoattractant protein-1, stromal cell-derived factor-1, vascular endothelial growth factor-A, endothelin-1, and tumor growth factor-beta(1) in pulmonary vessel wall cells, either directly or indirectly via signals from hypoxic lung epithelial cells, may be a critical first step in the recruitment of circulating leukocytes to the pulmonary circulation. In addition, hypoxic stress appears to induce release of increased numbers of monocytic progenitor cells from the bone marrow, and these cells may have upregulated expression of receptors for the chemokines produced by the lung circulation, which thus facilitates their specific recruitment to the pulmonary site. Once present, macrophages/fibrocytes may exert paracrine effects on resident pulmonary vessel wall cells stimulating proliferation, phenotypic modulation, and migration of resident fibroblasts and smooth muscle cells. They may also contribute directly to the remodeling process through increased production of collagen and/or differentiation into myofibroblasts. In addition, they could play a critical role in initiating and/or supporting neovascularization of the pulmonary artery vasa vasorum. The expanded vasa network may then act as a conduit for further delivery of circulating mononuclear cells to the pulmonary arterial wall, creating a feedforward loop of pathological remodeling. Future studies will need to determine the mechanisms that selectively induce leukocyte/fibrocyte recruitment to the lung circulation under hypoxic conditions, their direct role in the remodeling process via production of extracellular matrix and/or differentiation into myofibroblasts, their impact on the phenotype of resident smooth muscle cells and adventitial fibroblasts, and their role in the neovascularization observed in hypoxic pulmonary hypertension.

5-Hydroxytryptamine-induced proliferation of pulmonary artery smooth muscle cells are extracellular signal-regulated kinase pathway dependent

AIM

To investigate the effect of 5-hydroxytryptamine transporter (5-HTT) inhibitor fluoxetine and antisense oligodeoxynucleotide (ODN) to extracelluar signal-regulated kinases (ERKs) on pulmonary arterial smooth muscle cells (PASMCs) proliferation induced by 5-HT.

METHODS

Liposomal transfection was used to introduce ODNs to ERK1/2 into cultured rat PASMCs and the transfection efficiency was measured by observing the uptake of the fluorecein isothiocynate (FITC)-labeled antisense ODN in PASMCs. The effects of 5-HTT selective inhibitor fluoxetine and ODNs on the proliferation of PASMCs were evaluated by cell number counting and cell cycle analysis, and measured by microculture tetrazolium (MTT) assay and flow cytometry (FCM), respectively.

RESULTS

Liposomes mediated the transfection of ODNs into PASMCs with high efficiency. MTT assay showed fluoxetine (10 micromol/L, 1 micromol/L, and 100 nmol/L) concentration dependently inhibited the proliferation of PASMCs induced by 5-HT (1 micromol/L) in vitro. The proliferation rate of PASMCs by 5-HT was significantly inhibited by pretreatment with ERK1/2 antisense ODN (0.2 micromol/L) from 251%+/-18% to 86%+/-5% (P<0.01). Flow cytometric analysis of cell cycle distribution showed that the increase of 5-HT induced S phase fraction (SPF) and proliferation index (PI) were significantly inhibited by fluoxetine (1 micromol/L) or antisense ODN with SPF from 36%+/-4% to 26%+/-3% and 24%+/-4%, and PI from 34%+/-2% to 29%+/-2% and 24%+/-2%, respectively.

CONCLUSION

5-HTT mediates the mitogenic effect of 5-HT on PASMCs and the proliferation of PASMCs induced by 5-HT is dependent on ERKs signal pathway.

The pathogenesis of pulmonary hypertension in haemodialysis patients via arterio-venous access

BACKGROUND

We recently have shown a high incidence of unexplained pulmonary hypertension (PHT) in end-stage renal disease (ESRD) patients on chronic haemodialysis (HD) therapy via arterio-venous (A-V) access. This study evaluated the possibility that PHT in these patients is triggered or aggravated by chronic HD via surgical A-V access, and the role of endothelin-1 (ET-1) and nitric oxide (NO) in this syndrome.

METHODS

Forty-two HD patients underwent clinical evaluation. Pulmonary artery pressure (PAP) was evaluated using Doppler echocardiography. Levels of ET-1 and NO metabolites in plasma were determined before and after the HD procedure and were compared between subgroups of patients with and without PHT.

RESULTS

Out of 42 HD patients studied, 20 patients (48%) had PHT (PAP = 46+/-2; range 36-82 mmHg) while the rest had a normal PAP (29+/-1 mmHg) (P<0.0001). HD patients with PHT had higher cardiac output compared with those with normal PAP (6.0+/-1.2 vs 5.2+/-0.9 l/min, P<0.034). HD patients, with or without PHT, had elevated plasma ET-1 levels compared with controls (1.6+/-0.7 and 2.4+/-0.8 fmol/ml vs 1.0+/-0.2, P<0.05) that remained unchanged after the HD procedure. HD patients without PHT and control subjects showed similar basal plasma levels of NO2 + NO3 (24.2+/-5.2 vs 19.7+/-3.1 microM, P>0.05) that was significantly higher compared with HD patients with PHT (14.3+/-2.3 microM, P<0.05). HD therapy caused a significant increase in plasma NO metabolites that was greater in patients without PHT (from 24.2+/-5.2 to 77.1+/-9.6 microM, P<0.0001, and from 14.3+/-2.3 to 39.9+/-11.4 microM, P<0.0074, respectively). Significant declines in PAP (from 49.8+/-2.8 to 38.6+/-2.2 mmHg, P<0.004) and cardiac output (CO) (from 7.6+/-0.6 to 6.1+/-0.3 l/min, P<0.03) were found in 11 HD patients with PHT that underwent successful transplantation. Similarly, temporary closure of the A-V access by a sphygmomanometer in eight patients with PHT resulted in a transient decrease in CO (from 6.4+/-0.6 to 5.3+/- 0.5 l/min, P = 0.18) and systolic PAP (from 47.2+/-3.8 to 34.6+/-2.8 mmHg, P<0.028).

CONCLUSIONS

This study demonstrates a high prevalence of PHT among patients with ESRD on chronic HD via a surgical A-V fistula. In view of the vasodilatory and antimitogenic properties of NO, it is possible that the attenuated basal and HD-induced NO production in patients with PHT contributes to the increased pulmonary vascular tone. Furthermore, the partial restoration of normal PAP and CO in HD patients that underwent either temporal A-V shunt closure or successful transplantation indicates that excessive pulmonary blood flow is involved in the pathogenesis of the disease.

[Physiopathology of pulmonary arterial hypertension. Cellular and molecular aspects]

The combined effects of vasoconstriction, remodelling of the pulmonary vessel walls and in situ thrombosis contribute to the increase in pulmonary vascular resistance during pulmonary arterial hypertension. Vascular remodelling involves all the sheaths of the vessel wall and all the cell types of which it is composed (endothelial cells, smooth muscle cells, fibroblasts, inflammatory cells and platelets). Excessive vasoconstriction has been related to a defect in the function of expression of the potassium channels and endothelial dysfunction. This leads to chronic insufficiency in the production of vasodilators, notably nitrogen monoxide and prostacyclin and the excessive production of vasoconstrictors such as endotheline-1. These defects contribute to the increase in vascular tonus and pulmonary vascular remodelling and represent pertinent pharmacological targets. Certain growth factors, including those of the super-family of transforming growth factor beta, angiopoietine-1 and serotonin, may play a part in the pathogenesis of pulmonary arterial hypertension.

A role for platelet-derived growth factor beta-receptor in a newborn rat model of endothelin-mediated pulmonary vascular remodeling

Newborn rats exposed to 60% O2 for 14 days develop endothelin (ET)-1-dependent pulmonary hypertension with vascular remodeling, characterized by increased smooth muscle cell (SMC) proliferation and medial thickening of pulmonary resistance arteries. Using immunohistochemistry and Western blot analyses, we examined the effect of exposure to 60% O2 on expression in the lung of receptors for the platelet-derived growth factors (PDGF), which are implicated in the pathogenesis of arterial smooth muscle hyperplasia. We observed a marked O2-induced upregulation of PDGF-alpha and -beta receptors (PDGF-alphaR and -betaR) on arterial smooth muscle. This led us to examine pulmonary vascular PDGF receptor expression in 60% O2-exposed rats given SB-217242, a combined ET receptor antagonist, which we found prevented the O2-induced upregulation of PDGF-betaR, but not PDGF-alphaR, on arterial smooth muscle. PDGF-BB, a major PDGF-betaR ligand, was found to be a potent in vitro inducer of hyperplasia and DNA synthesis in cultured pulmonary artery SMC from infant rats. A critical role for PDGF-betaR ligands in arterial SMC proliferation was confirmed in vivo using a truncated soluble PDGF-betaR intervention, which attenuated SMC proliferation induced by exposure to 60% O2. Collectively, these data are consistent with a major role for PDGF-betaR-mediated SMC proliferation, acting downstream of increased ET-1 in a newborn rat model of 60% O2-induced pulmonary hypertension.

Transforming Growth Factor-β Receptor Mutations and Pulmonary Arterial Hypertension in Childhood

Background— Pulmonary arterial hypertension (PAH) is a potentially fatal vasculopathy that can develop at any age. Adult-onset disease has previously been associated with mutations in BMPR2 and ALK-1 . Presentation in early life may be associated with congenital heart disease but frequently is idiopathic.

Methods and Results— We performed mutation analysis in genes encoding receptor members of the transforming growth factor-β cell-signaling pathway in 18 children (age at presentation <6 years) with PAH. Sixteen children were initially diagnosed with idiopathic PAH and 2 with PAH in association with congenital heart defects. Germ-line mutations were observed in 4 patients (22%) (age at disease onset, 1 month to 6 years), all of whom presented with idiopathic PAH. The BMPR2 mutations (n=2, 11%) included a partial gene deletion and a nonsense mutation, both arising de novo in the proband. Importantly, a missense mutation of ALK-1 and a branch-site mutation of endoglin were also detected. Presenting clinical features or progression of pulmonary hypertension did not distinguish between patients with mutations in the different genes or between those without mutations.

Conclusions— The cause of PAH presenting in childhood is heterogeneous in nature, with genetic defects of transforming growth factor-β receptors playing a critical role.

PDGF and IL-1β Upregulate Cofilin and LIMK2 in Canine Cultured Pulmonary Artery Smooth Muscle Cells

Actin cytoskeleton reorganization is regulated by various actin-binding proteins. Cofilin is the principal filament-depolymerizing protein, whose activity is reduced upon phosphorylation by LIMK. Thus, LIMK and cofilin comprise a signal transduction module regulating actin turnover and myogenic tone in healthy vasculature. Novel functions of smooth muscle cells (SMCs) in the hypertensive pulmonary artery, such as increased motility and proliferation, are supported by the actin cytoskeleton. We therefore hypothesized that bioactive peptides that affect these SMC functions may also result in an upregulation of LIMK and cofilin expression. Semiquantitative RT-PCR and immunoblotting indicated that LIMK2 and cofilin mRNA and protein expression is upregulated in canine pulmonary artery SMCs (PASMCs) exposed to PDGF or IL-1beta (10 ng/ml). Inhibition of ERK MAPKs (U-0126, 10 muM) or p38 MAPK (PD-169316, 10 muM), but not PI3Ks (LY-294002, 50 muM), reduced LIMK2 and cofilin gene expression stimulated by PDGF or IL-1beta. Inhibition of ROCK (Y-27632, 10 muM) reduced only the IL-1beta-stimulated LIMK2 and cofilin expression. These novel observations in PASMCs indicate that LIMK2 and cofilin expression can be induced by PDGF or IL-1beta. This parallel upregulation of LIMK2 and cofilin may have potentially broad functional significance for the progress of pulmonary artery disease.

Differential effects of TGF-β1 and BMP-4 on the hypoxic induction of cyclooxygenase-2 in human pulmonary artery smooth muscle cells

Chronic hypoxia-induced pulmonary hypertension results partly from proliferation of smooth muscle cells in small peripheral pulmonary arteries. Previously, we demonstrated that hypoxia modulates the proliferation of human peripheral pulmonary artery smooth muscle cells (PASMCs) by induction of cyclooxygenase-2 (COX-2) and production of antiproliferative prostaglandins ( 55 ). The transforming growth factor (TGF)-β superfamily plays a critical role in the regulation of pulmonary vascular remodeling, although to date an interaction with hypoxia has not been examined. We therefore investigated the pathways involved in the hypoxic induction of COX-2 in peripheral PASMCs and the contribution of TGF-β1 and bone morphogenetic protein (BMP)-4 in this response. In the present study, we demonstrate that hypoxia induces activation of p38MAPK, ERK1/2, and Akt in PASMCs and that these pathways are involved in the hypoxic regulation of COX-2. Whereas inhibition of p38MAPKor ERK1/2 activity suppressed hypoxic induction of COX-2, inhibition of the phosphoinositide 3-kinase pathway enhanced hypoxic induction of COX-2. Furthermore, exogenous TGF-β1 induced COX-2 mRNA and protein expression, and our findings demonstrate that release of TGF-β1 by PASMCs during hypoxia contributes to the hypoxic induction of COX-2 via the p38MAPKpathway. In contrast, BMP-4 inhibited the hypoxic induction of COX-2 by an MAPK-independent pathway. Together, these findings suggest that the TGF-β superfamily is part of an autocrine/paracrine system involved in the regulation of COX-2 expression in the distal pulmonary circulation, and this modulates hypoxia-induced pulmonary vascular cell proliferation.

BMP4 inhibits proliferation and promotes myocyte differentiation of lung fibroblasts via Smad1 and JNK pathways

Fibroblast proliferation, differentiation, and migration contribute to the characteristic pulmonary vascular remodeling seen in primary pulmonary hypertension (PPH). The identification of mutations in the bone morphogenetic protein type II receptor (BMPRII) in PPH have led us to question what role BMPRII and its ligands play in pulmonary vascular remodeling. Thus, to further understand the functional significance of BMPRII in the pulmonary vasculature, we examined the expression of TGF-beta superfamily receptors in human fetal lung fibroblasts (HFL) and investigated the role of BMP4 on cell cycle regulation, fibroblast proliferation, and differentiation. Furthermore, signaling pathways involved in these processes were examined. HFL expressed BMPRI and BMPRII mRNA and demonstrated specific I(125)-BMP4 binding sites. BMP4 inhibited [(3)H]thymidine incorporation and proliferation of HFL; protein expression was increased for the cell cycle inhibitor p21 and reduced for the positive regulators cyclin D and cdk2 by BMP4. BMP4 induced differentiation of HFL into a smooth muscle cell phenotype since protein expression of alpha-smooth muscle actin and smooth muscle myosin was increased. Furthermore, p38(MAPK), ERK1/2, JNK, and Smad1 were phosphorylated by BMP4. Using specific MAPK inhibitors, a dominant negative Smad1 construct, and Smad1 siRNA, we found that the antiproliferative and prodifferentiation effects of BMP4 were Smad1 dependent with JNK also contributing to differentiation. Because failure of Smad phosphorylation is a major feature of BMPRII mutations, these results imply that BMPRII mutations may promote the expansion of fibroblasts resistant to the antiproliferative, prodifferentiation effects of BMPs and suggest a mechanism for the vascular obliteration seen in familial PPH.

cAMP phosphodiesterase inhibitors potentiate effects of prostacyclin analogs in hypoxic pulmonary vascular remodeling

We investigated the effects of prostacyclin analogs and isoform-selective phosphodiesterase (PDE) inhibitors, alone and in combination, on pulmonary vascular remodeling in vitro and in vivo. Vascular smooth muscle cells (VSMC) isolated from pulmonary (proximal and distal) and systemic circulations demonstrated subtle variations in expression of PDE isoform mRNA. However, using biochemical assays, we found PDE3 and PDE4 isoforms to be responsible for the majority of cAMP hydrolysis in all VSMC. In growth assays, the prostacyclin analogs cicaprost and iloprost inhibited mitogen-induced proliferation of VSMC in a cAMP-dependent manner. In addition, isoform-selective antagonists of PDEs 1, 3, or 4 inhibited VSMC proliferation, an effect that synergized with the effect of prostacyclin analogs. The inhibitory effects were greater in cells isolated from pulmonary circulation. In an in situ perfused rat lung preparation, administration of prostacyclin analogs or the PDE inhibitors vinpocetine (PDE1), cilostamide (PDE3), or rolipram (PDE4), but not EHNA (PDE2), attenuated acute hypoxic vasoconstriction (HPV). Combinations of agents led to a greater reduction in HPV. Furthermore, during exposure to hypoxia for 13 days, Wistar rats were treated with iloprost, rolipram, cilostamide, or combinations of these agents. Compared with normoxic controls, hypoxic animals developed pulmonary hypertension and distal pulmonary artery muscularization. These parameters were attenuated by iloprost+cilostamide, iloprost+rolipram, and cilostamide+rolipram but were not significantly affected by single agents. Together, these findings provide a greater understanding of the role of cAMP PDEs in VSMC proliferation and provide rationale for combined use of prostacylcin analogs plus PDE3/4 inhibitors in treatment of pulmonary vascular remodeling.

Increased arginase II and decreased NO synthesis in endothelial cells of patients with pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH), a fatal disease of unknown etiology characterized by impaired regulation of pulmonary hemodynamics and vascular growth, is associated with low levels of pulmonary nitric oxide (NO). Based upon its critical role in mediating vasodilation and cell growth, decrease of NO has been implicated in the pathogenesis of PAH. We evaluated mechanisms for low NO and pulmonary hypertension, including NO synthases (NOS) and factors regulating NOS activity, i.e. the substrate arginine, arginase expression and activity, and endogenous inhibitors of NOS in patients with PAH and healthy controls. PAH lungs had normal NOS I-III expression, but substrate arginine levels were inversely related to pulmonary artery pressures. Activity of arginase, an enzyme that regulates NO biosynthesis through effects on arginine, was higher in PAH serum than in controls, with high-level arginase expression localized by immunostaining to pulmonary endothelial cells. Further, pulmonary artery endothelial cells derived from PAH lung had higher arginase II expression and produced lower NO than control cells in vitro. Thus, substrate availability affects NOS activity and vasodilation, implicating arginase II and alterations in arginine metabolic pathways in the pathophysiology of PAH.

Cellular and molecular pathobiology of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) has a multifactorial pathobiology. Vasoconstriction, remodeling of the pulmonary vessel wall, and thrombosis contribute to increased pulmonary vascular resistance in PAH. The process of pulmonary vascular remodeling involves all layers of the vessel wall and is complicated by cellular heterogeneity within each compartment of the pulmonary arterial wall. Indeed, each cell type (endothelial, smooth muscle, and fibroblast), as well as inflammatory cells and platelets, may play a significant role in PAH. Pulmonary vasoconstriction is believed to be an early component of the pulmonary hypertensive process. Excessive vasoconstriction has been related to abnormal function or expression of potassium channels and to endothelial dysfunction. Endothelial dysfunction leads to chronically impaired production of vasodilators such as nitric oxide and prostacyclin along with overexpression of vasoconstrictors such as endothelin (ET)-1. Many of these abnormalities not only elevate vascular tone and promote vascular remodeling but also represent logical pharmacological targets. Recent genetic and pathophysiologic studies have emphasized the relevance of several mediators in this condition, including prostacyclin, nitric oxide, ET-1, angiopoietin-1, serotonin, cytokines, chemokines, and members of the transforming-growth-factor-beta superfamily. Disordered proteolysis of the extracellular matrix is also evident in PAH. Future studies are required to find which if any of these abnormalities initiates PAH and which ones are best targeted to cure the disease.

Prostacyclin and its analogues in the treatment of pulmonary hypertension

Prostacyclin and its analogues (prostanoids) are potent vasodilators and possess antithrombotic and antiproliferative properties. All of these properties help to antagonize the pathological changes that take place in the small pulmonary arteries of patients with pulmonary hypertension. Indeed, several prostanoids have been shown to be efficacious to treat pulmonary hypertension, while the main mechanism underlying the beneficial effects remains unknown. There are indications of beneficial combination effects of prostaglandins and phosphodiesterase inhibitors and endothelin receptor antagonists. This speaks in favor of combination therapies for pulmonary hypertension in the future. The mode of application of prostanoids used in randomized controlled studies has been quite variable: continuous i.v. infusion of prostacyclin, continuous s.c. infusion of treprostinil, p.o. application of beraprost, and inhaled application of iloprost. In addition, the applied doses were quite different, ranging from 0.25 ng/kg/min for inhaled iloprost to 30-50 ng/kg/min for i.v. prostacyclin. While the principal pharmacological properties of all prostanoids are very similar due to a main action on IP receptors, there are considerable differences in pharmacokinetics and metabolism, with half-lives of 2 min for prostacyclin and about 34 min for treprostinil for i.v. infused drugs and half-lives of about 85 min for s.c. infused treprostinil. In addition, the adverse effects largely depend on the doses used and the mode of application, although there is great variability between subjects. It remains to be determined which patients will profit most from which substance (or combination) and mode of application.

Paired-related homeobox gene Prx1 is required for pulmonary vascular development

Herein, we show that the paired-related homeobox gene, Prx1, is required for lung vascularization. Initial studies revealed that Prx1 localizes to differentiating endothelial cells (ECs) within the fetal lung mesenchyme, and later within ECs forming vascular networks. To begin to determine whether Prx1 promotes EC differentiation, fetal lung mesodermal cells were transfected with full-length Prx1 cDNA, resulting in their morphological transformation to an endothelial-like phenotype. In addition, Prx1-transformed cells acquired the ability to form vascular networks on Matrigel. Thus, Prx1 might function by promoting pulmonary EC differentiation within the fetal lung mesoderm, as well as their subsequent incorporation into vascular networks. To understand how Prx1 participates in network formation, we focused on tenascin-C (TN-C), an extracellular matrix (ECM) protein induced by Prx1. Immunocytochemistry/histochemistry showed that a TN-C-rich ECM surrounds Prx1-positive pulmonary vascular networks both in vivo and in tissue culture. Furthermore, antibody-blocking studies showed that TN-C is required for Prx1-dependent vascular network formation on Matrigel. Finally, to determine whether these results were relevant in vivo, we examined newborn Prx1-wild-type (+/+) and Prx1-null (-/-) mice and showed that Prx1 is critical for expression of TN-C and lung vascularization. These studies provide a framework to understand how Prx1 controls EC differentiation and their subsequent incorporation into functional pulmonary vascular networks.

Homeobox genes in pulmonary vascular development and disease

Determining how the pulmonary vascular system is formed, maintained, or disrupted during development and disease represents a major challenge in contemporary lung biology. Whereas it is appreciated that cellular proliferation, differentiation, migration, and apoptosis need to be carefully controlled in order to attain pulmonary vascular homeostasis, knowledge of the underlying cellular and molecular mechanisms involved remains surprisingly limited. Because homeobox genes represent master regulators of organogenesis and tissue patterning, it is likely that these transcription factors play a critical role in the formation of blood vessels within the lung, as well as in pathologic states in which the highly ordered structure of the pulmonary vascular tree is compromised. The aim of this review is to discuss some of the known functions of homeobox genes in the vasculature, and to extrapolate these findings to their potential roles in developing and diseased pulmonary vessels.

Interleukin-1beta, transforming growth factor-beta1, and bradykinin attenuate cyclic AMP production by human pulmonary artery smooth muscle cells in response to prostacyclin analogues and prostaglandin E2 by cyclooxygenase-2 induction and downregulation of adenylyl cyclase isoforms 1, 2, and 4

Increased levels of inflammatory cytokines contribute to the pathophysiology of pulmonary hypertension. Prostacyclin (PGI2) analogues, which relax pulmonary vessels mainly through cAMP elevation, have a major therapeutic role. In this study, we show that prolonged incubation with bradykinin (BK), interleukin-1beta (IL-1beta), and transforming growth factor-beta1 (TGF-beta1) markedly impairs cAMP accumulation in human pulmonary artery smooth muscle cells in response to short-term incubation with prostaglandin E2 (PGE2) and the PGI2 analogues iloprost and carbaprostacyclin. A similar reduction in cAMP accumulation in response to a direct adenylyl cyclase activator, forskolin, suggested that the effect was attributable to downregulation of adenylyl cyclase. Reverse transcriptase-polymerase chain reaction studies showed downregulation of adenylyl cyclase isoforms 1, 2, and 4. The effect of IL-1beta, BK, and TGF-beta1 on cAMP levels was abrogated by the selective COX-2 inhibitor NS398. Furthermore, it was mimicked by prolonged incubation with the COX-2 product PGE2 and PGI2 analogues or the COX substrate arachidonic acid, suggesting that it was mediated by endogenous prostanoids produced by COX-2. Consistent with this, IL-1beta, BK, and TGF-beta1 all induced COX-2 and PGE2 release. These results show that BK, IL-1beta, and TGF-beta1 downregulate adenylyl cyclase in human pulmonary artery smooth muscle cells via COX-2 induction and prostanoid release. This suggests a novel mechanism whereby mediators and cytokines produced in pulmonary hypertension may impair the therapeutic effects of prostacyclin analogues such as iloprost and carbaprostacyclin.

Pulmonary artery remodeling in transposition of the great arteries: relevance for neoaortic root dilatation

OBJECTIVE

Transposition of the great arteries is currently treated by performing the arterial switch operation. Dilatation of the neoaortic root is a late complication with unknown cause. Samples of patients with untreated transposition of the great arteries and patients with normally related great arteries were compared to investigate a possible role for vascular remodeling in the dilatation process.

METHODS

Aortic and pulmonary artery vessel wall and sinus samples were taken from 20 untreated human heart specimens with transposition of the great arteries and 9 age-matched, normal, postmortem human heart specimens, divided into 2 groups according to age. Routine histology and immunohistochemical staining for smooth muscle cell differentiation markers alpha-smooth muscle actin, SM22, and calponin were performed.

RESULTS

This study revealed structural differences between the normal aorta and pulmonary artery in the early group, which became more pronounced in the late group. In the early stage in transposition of the great arteries, no marked differences were seen between the aorta and pulmonary artery. With increasing age, however, there was a pronounced down-regulation of all smooth muscle cell markers in the pulmonary artery.

CONCLUSIONS

There is a structural difference between the normal neonatal aorta and pulmonary artery. The great arteries in transposition of the great arteries differ from each other and from normal vessels, indicating a structural vascular difference in transposition of the great arteries. In the pulmonary artery and sinus of untreated transposition of the great arteries, there is a dedifferentiation of smooth muscle cells with increasing age that we could not correlate to altered flow. This structural abnormality might provide an explanation for the neoaortic root dilatation that has been reported as a late complication of the arterial switch operation.

Effects of bosentan on cellular processes involved in pulmonary arterial hypertension: do they explain the long-term benefit?

Pulmonary arterial hypertension is a rapidly progressing disease characterized by an over- expression of endothelin. In addition to its potent pulmonary vasoconstrictor effects, endothelin has been shown to produce many of the aberrant changes, such as hypertrophy, fibrosis, inflammation, and neurohormonal activation that underlie the shortened life span in pulmonary arterial hypertensive patients. The fact that endothelin expression correlates significantly with disease severity and outcome in these patients suggests that endothelin, through binding to both ETA and ETB receptor subtypes, is a key causative agent in the pathophysiology of pulmonary arterial hypertension. The orally active dual endothelin receptor antagonist bosentan competitively antagonizes the binding of endothelin to both endothelin receptor subtypes with high affinity and specificity. In animal models relevant for the pathophysiology of pulmonary hypertension, bosentan not only causes selective pulmonary vasodilation, but also prevents vascular hypertrophy and cardiac remodeling, attenuates pulmonary fibrosis, decreases vascular inflammation, and blunts neurohormonal activation. These experimental data may explain the effects on disease progression and the long-term benefit observed with bosentan in pulmonary arterial hypertension.